This invention relates to hard and tough tools and knives capable of holding an edge and resisting corrosion better than conventional materials used for cutting instruments, and more particularly to processes for making cutting instruments of Type 60 Nitinol to produce tools and knives that are hard, tough, and elastic, and which are virtually immune to corrosion.
The development of Damascus steel in the 5th century produced a hard and tough material that was, and still is, prized for knives and swords. However, the process of manufacturing Damascus steel is arduous and expensive, and the material is susceptible to rusting and other corrosion, so it must be kept oiled and otherwise protected from corrosive influences. Because of its susceptibility to corrosion and the need to keep it oiled to prevent such corrosion, Damascus steel is not suitable for food preparation, skinning game or other meat cutting applications, so its usefulness in the real world is limited. Its primary application is for display knives and swords because of the distinctive banded appearance of the material.
Since the development of Damascus steel, the most important commercial development in the field of cutlery materials in the last century has been corrosion resistant steel, more commonly (although inaccurately) called xe2x80x9cstainless steelxe2x80x9d. Stainless steel is characterized by the inclusion of chromium and sometimes nickel in the composition which makes it significantly more corrosion resistant than other types of steels, and as a result is now very widely used in most cutlery. However, in order to obtain the hardness necessary for retaining a decent cutting edge, the material must be well above 400 on the Brinell scale, or 42 on the Rockwell C scale, and preferably above 500 Brinell. Compositions of corrosion resistant steel having high hardness have been developed, such as 440C which can be heat treated to a hardness of about 56 on the Rockwell C scale. That is an adequate hardness for retaining a good cutting edge, but this material also has a high carbon content of about 1.5% and is difficult to machine. The high carbon content results in reduced corrosion resistance, resulting in a tendency for the cutting edges of knives made from this material to become dull because the thin cusp of the cutting edge corrodes away, leaving a rounded edge. This is particularly true for environments containing the chlorine ion, such as sea water and chlorine cleaning solutions used in food preparation areas. The cost of knives made from 440C stainless steel is higher than knives made from other material because processing the material and machining the blade shape is more difficult than it is for other knife materials. Finally, knives made of 440C stainless steel have a tendency to lose their luster over a relatively short time because of tarnishing, stains and corrosion, resulting in a knife blade with an unattractive, dingy appearance which customers dislike.
FDA requirements for knives used in meat cutting and fish processing operations have very stringent corrosion resistance standards which limit the carbon content of the stainless steel to no more than 0.10%. Corrosion resistant steels are available that meet these requirements and knives made from them do indeed exhibit adequate corrosion resistance for safe use in meat cutting and fish processing facilities. However, these materials are soft, less than 200 on the Brinell scale, and knives made from these materials dull quickly and must be sharpened continually. As a consequence, these knives last only a short time before they are sharpened away and are discarded. This industry has long needed a knife that is approved for use around meat, poultry and fish by the U.S. FDA and would remain sharp for long periods of constant use without sharpening.
In the field of cutting instruments other than cutlery, the most significant commercially development in the last century has been sintered carbides of silicon, tungsten and titanium in a metallic matrix of cobalt or other toughening metals. Carbides are very hard and hold a cutting edge better than most other materials, but they are brittle and tend to shatter when stressed beyond their yield strength. When designed properly and used within the intended parameters of workpiece hardness and cutter feeds and speeds, carbide cutting inserts provide long life to cutting tools and are very widely used throughout the industry. However, even with improvements to the matrix material, the bonding interface between the carbides and the matrix material, and the manufacturing processes, no carbide metal matrix composite materials have been developed that would be suitable for cutlery, and the material remains so brittle that its use must be carefully controlled to prevent shattering of the cutting instrument if the feed speed, cutter speed or the hardness or toughness of the workpiece exceeds the stress limitations of the carbide cutter.
Chipper and shredder blades, lawn mower blades, and brush cutter blades are notorious for their short blade life. These blades have high velocity and often encounter hard materials such as rocks and metal debris in their operation, so they must be made malleable. If they were made hard to retain a long blade life, they would be brittle and subject to catastrophic failure in the event of impact with a rock or the like which could shatter the blade or initiate a crack which could grow through the blade material and rupture at some unpredictable time in the near future. The malleable material does not crack or shatter like the harder material would, but it is also relatively soft and the blade edge quickly becomes rounded in ordinary use. The rounded edge cuts slower, requires more energy to cut, and cuts with a ragged edge rather than a clean edge, essentially breaking rather than cutting. In large chipping operations such as slash chipping in logging operations, the conventional chipper blades must be changed frequently, resulting in lengthy and inefficient downtime and idling of the operators. A blade material for machines of this nature that is hard and holds an edge, and is tough instead of brittle like conventional hard materials, would be extremely welcome to owners and operators of these machines. These same considerations also apply to machines such as stump grinders and road scarifying machines that are actually expected to involve contact with the ground or with rocks.
Many hand tools such as axes, splitting mauls and picks have the same malleability requirements that blades for chippers and shredders and that sort of machine have. The cutting or leading edge must be made malleable enough to yield or roll over on impact with a hard material so that it does not chip or break and produce flying metal fragments that would be dangerous to the user or bystanders, especially to their eyes.
Tools such as pruning shears, clippers, and saws, grafting knives, and chain saws used on green plants often get gummed up with plant sap that sticks tenaciously and is very difficult to clean off the tool. The sticky sap interferes with smooth cutting by the tool since it prevents the tool blade from sliding smoothly through the cut. The sap also promotes corrosion of the tool surfaces which makes even more difficult the task of cleaning the old sap off the tool. The corrosion around the tool cutting edges dulls the cutting edges and also gums up the sharpening tools.
Medical cutting instruments such as chisels, files, and scalpels currently in use are made primarily from 300 series stainless steels, primarily because of its corrosion resistance and tendency to bend rather than chip if the instrument encounters bone. However, the 300 series stainless steels are so soft that the instruments quickly become dull and must be replaced with sharp instruments. Scalpel sharpness is very important to a surgeon, and it is commonplace in lengthy operations for numerous scalpels to be used and discarded in the course of the operation.
In the 1960""s, the Naval Ordnance Laboratory in White Oak, Md. invented an intermetallic compound of nickel and titanium which they named xe2x80x9cNitinolxe2x80x9d. One form of that material, which they named xe2x80x9cType 60 Nitinolxe2x80x9d, has a composition of 57-63% by weight nickel and the balance titanium. The Navy was interested in this material because it was nonmagnetic and potentially useful to the Navy Seal commandos in knives to defuse mines or cut anchoring cables on magnetic mines during the Vietnam war. The Navy had a small quantity of this material made and an experimental program was initiated to fashion it into knives for its Seal commandos, but the material proved to be so difficult to machine that the contractor was unable to produce more than a few prototypes and no further knives were made, despite the desirable attributes of the knife.
Thus, there has long been a need for a cutting instrument that is corrosion proof, hard, flexible, and tough, and can be polished to a high long lasting luster. This cutting instrument would have the ability to hold an edge for a long period, even in corrosive environments such as salt water and industrial chemicals, and it would be FDA approved for use in meat cutting and fish processing facilities, as well as in hospital operating rooms. High production rate processes for making such a knife at reasonable costs also have been long needed by the industry and, once adopted, will militate for the replacement of stainless steel by Type 60 Nitinol for all but the cheapest cutting instruments.
Accordingly, it is an object of this invention to provide improved processes for making a cutting instrument from Type 60 Nitinol. Another object of this invention is to provide an improved cutting instrument having a monolithic blade with a surface finish smoother than about 20 microinches and an edge hardness exceeding 55 on the Rockwell C scale. Yet another object of this invention is to provide a cutting instrument that is immune to corrosion from common corrosive agents, including ocean saltwater.
These and other objects are attained in a cutting instrument having a blade made from a blank cut from a plate or strip of Type 60 Nitinol. The plate or strip, having a thickness of about 0.010xe2x80x3-0.500xe2x80x3, is first sandblasted to remove hard oxides that form during hot rolling of the plate. The blade blank is cut out of the plate using a laser cutter, abrasive waterjet or wire electron discharge machining. The blank is flattened to remove any curvature that may remain from the rolling operation, and is surface ground to a depth of about 0.001 to 0.005 inches top and bottom to remove surface imperfections in the plate. An edge is ground into the blade blank using a PCBN or diamond grinding wheel or belt and a liberal flooding of coolant to help capture the Nitinol particles. The surface of the blade can be polished to a lustrous surface finish, if desired, smoother than 2 microinches RMS, using a diamond grit abrasive polishing compound.